Element having fine periodic structure, and optical member, optical system, and optical device having element

ABSTRACT

An element includes a periodic structure formed on a surface of a base member and having a period smaller than a wavelength of incident light and an additional layer added on the periodic structure. The additional layer has a refractive index lower than that of a material for the periodic structure formed on the surface of the base member.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an element having a periodicstructure with a period smaller than the wavelength of incident light,an optical element, optical system, and optical device using theelement.

[0003] 2. Related Background Art

[0004] When light is transmitted through a boundary between air andglass having different refractive indexes, Fresnel reflection occurs. Ingeneral, when light is incident on glass, about 4% of the light isreflected by Fresnel reflection. An optical device such as a camera orliquid crystal projector uses many optical elements such as lenses andprisms. For this reason, in order to obtain desired optical performance,transmittance must be increased by reducing Fresnel reflection by somemeans.

[0005] As a technique generally used to reduce Fresnel reflection, atechnique of obtaining an antireflection function by adding alow-refractive-index material such as MgF₂ or SiO₂ having an appropriatethickness onto the surface of an optical element is used.

[0006] In the case of a single layer antireflection film, the thicknessof a single layer film can be designed by using equations given below:

n=(n _(i) ×n _(s))^(½)  (1)

nd cos θ=λ/4×(2m−1)  (2)

[0007] where n_(i) is the refractive index of an incident-side material,n_(s) is the refractive index of a substrate-side material, n is therefractive index of a thin film material added on the substrate, d isthe film thickness, θ is the incidence angle, λ is the designwavelength, and m is an integer.

[0008] By designing the refractive indexes of materials and optical filmthickness to satisfy equations (1) and (2), the reflectance at thedesign wavelength λ can be reduced to zero. In general, however, thereis no combination of a substrate material and thin film material thatperfectly satisfy equation (1), and hence the reflectance rarely becomeszero at the design wavelength.

[0009] When the reflectance is to be further reduced in a wide range,higher antireflection effect can be obtained by stacking a thin film.

[0010] The following problems are posed in an antireflection film usinga low-refractive-index material:

[0011] (a) Low-refractive-index materials that can be used for anantireflection film are limited.

[0012] (b) Depending on the affinity between a substrate material and alow-refractive-index material, a desired antireflection film cannot beadded.

[0013] (c) The antireflection film may deteriorate depending onenvironmental changes such as temperature and humidity changes.

[0014] Optical elements having fine periodic structures have beenstudied, and an antireflection function has been known as acharacteristic of the optical elements having the fine periodicstructures. More specifically, an antireflection function is realized byforming a fine periodic structure with a period smaller than thewavelength of incident light (e.g., about ½ to {fraction (1/10)} thewavelength of incident light) on a substrate.

[0015] A substrate having a rectangular fine periodic structure exhibitsa reduction in reflectance (scalar calculation) as compared with asubstrate without any fine structure. Obviously, by forming a fineperiodic structure on the surface of a substrate, the sameantireflection effect as that obtained by a thin film using alow-refractive-index material can be obtained. In addition, since a fineperiodic structure is formed on a substrate, no limitations are imposedon low-refractive-index materials to be used, and freer design isallowed as compared with the case where a thin film is used.

[0016] According to a conventional optical element, an antireflectionfunction is attained by appropriately controlling the thickness of athin material having a refractive index lower than that of a substratematerial and formed on the surface of the substrate. An antireflectionelement having a fine periodic structure with a period smaller than thewavelength of incident light exhibits an antireflection function similarto that of the conventional thin antireflection film.

[0017] In general, a protection film is preferably added on the surfaceof the element having the fine periodic structure in consideration ofdurability, i.e., protection against environmental changes such astemperature and humidity changes, protection against physical damages,and the like.

[0018] Japanese Patent Application Laid-open No. 11-48355 discloses anoptical element having a protection film formed on a fine unevenpattern, its mold, and a method of manufacturing the element. The finalshape of this element after the formation of the protection film becomesa target fine uneven pattern.

[0019] This is a proposal about a technique of forming a protection filmshape added on a substrate, but no proposal is made about a technique ofdesigning a specific optimal protection film thickness and the opticalperformance of an optical element to which a protection film is added.

[0020] Japanese Patent Application Laid-open No. 11-174216 discloses aprotection film for improving diffraction efficiency in a high-frequencyregion and a method of manufacturing the protection film for adiffraction optical element manufactured by forming a protection film ona side surface of each diffraction element in a sawtooth shape orstepped shape and removing the protection film after the formation of anantireflection film.

[0021] However, there is no proposal about a protection film for anantireflection element having a fine periodic structure with a periodsmaller than the wavelength of incident light.

[0022] Japanese Patent Application Laid-open No. 11-305005 discloses anantireflection film that stably and greatly reduces normally reflectedlight produced at a light-transmitting optical medium interface withdifferent refractive indexes. However, a periodic structure is formed oncondition that its period is longer than the wavelength of incidentlight, and no proposal is made about an antireflection element having afine periodic structure with a period smaller than the wavelength ofincident light.

SUMMARY OF THE INVENTION

[0023] In order to solve the above problems, an element according to thepresent invention has a periodic structure with a period smaller thanthe wavelength of incident light (the shortest wavelength of light to beused) formed on the surface of a base member, and also has a layer whichhas a refractive index lower than that of the periodic structure formedon the surface of the base member and is added on the periodicstructure.

[0024] With this structure, the additional layer is made to function asa protection layer (or protection film) for the periodic structure onthe base member, thus protecting the periodic structure on the basemember against environmental changes such as temperature and humiditychanges and physical damages. In addition, an element having anexcellent antireflection function can be realized by the combination ofthe antireflection effect obtained by the fine periodic structure formedon the base member and the antireflection effect obtained because therefractive index of the additional layer is lower than that of the basemember.

[0025] In the present invention, the additional layer may have aperiodic structure with a period smaller than the wavelength of incidentlight.

[0026] With this structure, an antireflection effect can be obtained bythe periodic structure of the additional layer itself, in addition tothe above two antireflection effects, thereby realizing an elementhaving a better antireflection function.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 is a schematic view showing an antireflection elementaccording to the first embodiment of the present invention;

[0028]FIG. 2 is a sectional view of the antireflection element accordingto the first embodiment;

[0029]FIG. 3 is a graph showing the reflectance characteristics of theantireflection element according to the first embodiment;

[0030]FIG. 4 is a sectional view of an antireflection element accordingto the second embodiment of the present invention;

[0031]FIG. 5 is a graph showing the reflectance characteristics of theantireflection element according to the second embodiment;

[0032]FIG. 6 is a graph showing the incidence angle characteristics ofthe antireflection element according to the second embodiment;

[0033]FIG. 7 is a schematic view showing an optical element having anantireflection function according to the third embodiment of the presentinvention;

[0034]FIG. 8 is a view showing the arrangement of a scanning opticalsystem according to the fourth embodiment of the present invention;

[0035]FIG. 9 is a schematic view showing a single layer antireflectionfilm;

[0036]FIG. 10 is a graph showing the reflectance characteristics of thesingle layer antireflection film;

[0037]FIG. 11 is a sectional view of an antireflection element having afine periodic structure with a rectangular section;

[0038]FIG. 12 is a graph showing the reflectance characteristics of theantireflection element having the fine periodic structure with arectangular section;

[0039]FIG. 13 is a sectional view of an antireflection element having afine periodic structure with a triangle section; and

[0040]FIG. 14 is a graph showing the reflectance characteristics of theantireflection element having the fine periodic structure with atriangle section.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0041] The arrangement and reflectance characteristics of a generallyused antireflection film will be described first. The generally usedantireflection film is obtained by adding a film made of a materialhaving a refractive index lower than that of a substrate and anappropriate thickness on the substrate.

[0042]FIG. 9 is a schematic view of a general single layerantireflection film. FIG. 9 shows a substrate 3 and single layerantireflection film 92. In order to make this single layerantireflection film have an antireflection function, an optical filmthickness nd may be set to satisfy equation (2) with respect to a designwavelength λ.

[0043] Assume that the design wavelength λ is set to 0.5 μm and MgF₂(n=1.38) is used as a low-refractive-index material. Obviously, in thiscase, the optical film thickness nd is 125 nm.

[0044]FIG. 10 shows the reflectance obtained when an MgF₂ single layerfilm having an optical film thickness of 125 nm is added on a PMMA(polymethylmethacrylate) (n=1.492). Referring to FIG. 10, the abscissarepresents the wavelength; and the ordinate, the reflectance. In FIG.10, the solid line represents the reflectance characteristics with thesingle layer film, and the dashed line represents the reflectancecharacteristics of only the PMMA substrate without the reflection film.

[0045] As is apparent from FIG. 10, the reflectance near a wavelength of0.5 μm becomes minimum, exhibiting the design performance.

[0046] The arrangement and reflectance characteristics of anantireflection element obtained by forming a fine periodic structurewith a period smaller than the wavelength of incident light on asubstrate will be described next.

[0047] Note that in this embodiment, “smaller than the wavelength ofincident light” means that the period of the periodic structure issmaller than the wavelength of incident light against which anantireflection effect is to be obtained. If, for example, anantireflection effect is to be obtained against visible light(wavelength: 400 nm to 700 nm), the period of the periodic structure issmaller than the minimum wavelength, 400 nm.

[0048]FIG. 11 is a schematic view showing an optical element having afine periodic structure 2 with a rectangular section on the substrate 3.This fine periodic structure 2 has a periodic structure only in aone-dimensional direction (horizontal direction on the drawing surface)and a rectangular cross-sectional shape. This fine periodic structure 2can be formed by a technique like electron beam direct writing orreactive ion etching (RIE).

[0049] As a substrate material, PMMA (n=1.492) was used, and a latticeperiod Λ, lattice depth d1, and filling factor w/Λ representing theratio of substrate material in a lattice portion were respectively setto 0.2 μm, 0.1 μm, and 0.5.

[0050] When the lattice period Λ is almost equal to the wavelength ofincident light, strong polarization characteristics, strong wavelengthdependency, and occurrence of high-order diffracted light are observed,which are characteristic in a resonance region. This makes it difficultto attain desired antireflection performance. In addition, if thisstructure is applied to an actual optical system, high-order diffractedlight may cause stray light.

[0051] In order to prevent such high-order diffracted light, the latticeperiod Λ is preferably made to have a fine periodic structure with asize about ½ to {fraction (1/10)} the wavelength of incident light. Morespecifically, if visible light is used, the fine periodic structurepreferably has a size about ½ to {fraction (1/10)} the shortestwavelength, 400 nm (0.4 μm), i.e., about 0.2 μm to 0.04 μm.

[0052]FIG. 12 shows the reflectance of an optical element having such afine periodic structure. The reflectance was calculated by rigorouscoupled-wave analysis as a vector diffraction theory. Referring to FIG.12, the solid line represents the reflectance characteristics with thefine period structure, and the dashed line represents the reflectancecharacteristics of only the PMMA substrate without the fine periodstructure. Obviously, the reflectance decreases with the fine periodicstructure as compared with the reflectance without the fine periodicstructure. As described above, the reflectance of a substrate can bedecreased by forming a periodic structure with a period smaller than thewavelength of incident light on the surface of the substrate.

[0053] (First Embodiment)

[0054] An antireflection element according to the first embodiment ofthe present invention will be described next with reference to FIGS. 1and 2. FIG. 1 is a perspective view showing the schematic arrangement ofthe antireflection element in this embodiment. FIG. 2 is a sectionalview of this element.

[0055] In this embodiment, PMMA (n=1.492) is used as a material for asubstrate (base member) 3, and MgF₂ (n=1.38) is used as a material for aprotection layer (additional layer) 4. In addition, a lattice period(pitch) Λ is set to 0.2 μm; a lattice depth d1, 0.085 μm, a protectionlayer thickness d2, 0.085 μm; and filling factor w/Λ, 0.5.

[0056] Note that the thickness d2 of the protection layer 4 ispreferably equal to or smaller than the lattice depth dl from theviewpoint of ensuring the rigidity of the protection layer 4. Rigorouscoupled-wave analysis as a vector diffraction theory was used to designthe lattice depth and protection layer thickness. FIG. 3 shows thereflectance characteristics in this case. Referring to FIG. 3, theabscissa represents the wavelength; and the ordinate, the reflectance.

[0057] As shown in FIG. 3, in the case of the PMMA substrate only(without the fine periodic structure) indicated by the dashed line, areflectance of about 4% appears in the visible region due to Fresnelreflection.

[0058] If a single layer antireflection film (film material: MgF₂(n=1.38), optical film thickness: 125 nm) is formed on the PMMAsubstrate, the minimum reflectance can be suppressed to about 1.4% (areflectance of about 2% or less in the visible region).

[0059] If only a rectangular fine periodic structure is formed on thePMMA substrate, the minimum reflectance can be suppressed to about 0.2%(a reflectance of 1% or less in the visible region).

[0060] In the case of an antireflection element 1 having the protectionlayer 4 added on a fine periodic structure 2 with a rectangular sectionon the PMMA substrate 3, the minimum reflectance can be suppressed toabout 0.06% (a reflectance of about 0.5% in the visible region).

[0061] In the antireflection element 1 having the protection layer 4formed on the fine periodic structure 2, an excellent antireflectionfunction can be attained, which is difficult to realize by aconventional antireflection scheme based on only a single layer film andonly a fine periodic structure, by optimizing the shape of the fineperiodic structure 2 (lattice period, lattice depth, filling factor, andthe like) and the shape of the protection layer 4 (protection layerthickness, projection layer material, and the like). This makes itpossible to provide an antireflection element with a reflectance lowerthan that of the conventional element.

[0062] In this embodiment, a material having a refractive index lowerthan that of the substrate 3 is used for the protection layer 4.According to Fresnel reflection, the reflectance increases at aninterface where the reflectance change between the materials is large.For this reason, materials are preferably selected, which minimize thereflectance change between the material on the incident side (e.g., air)and the material of the protection layer 4 and between the material ofthe protection layer 4 and the material of the substrate 3.

[0063] Referring to FIG. 2, the apparent refractive index (to bereferred to as “effective refractive index” hereinafter) of a portion ofthe protection layer 4 having the layer thickness d2 (a portion having aperiodic structure between the PMMA substrate 3 and the air layer) canbe calculated by using an effective refractive index method like thatdisclosed in M. Born and E. Wolf, Principles of Optics.

[0064] It is preferable that the effective refractive index of theportion of the protection layer 4 be substantially equal to the averagevalue of the refractive index of the incident-side material and therefractive index of the substrate-side material (the refractive index ofair and the refractive index of the PMMA substrate having the fineperiodic structure 2).

[0065] In this embodiment, an effective refractive index n_(eff)(representing the average value of effective refractive indexes withrespect to TE and TM polarized light beams) of the portion of theprotection layer 4 is 1.223, which is almost equal to an averagerefractive index n_(ave) (=1.246) of air and PMMA.

[0066] As described above, by using a protection film material having anappropriate refractive index, an improvement in antireflectionperformance can be achieved as well as the protection of the substratematerial.

[0067] In the conventional single layer antireflection film, acombination of materials having ideal refractive indexes that cansatisfy equation (1) cannot be realized, it is difficult to form asingle layer antireflection film with a low reflectance.

[0068] In contrast to this, according to this embodiment, the effectiverefractive index is controlled by the protection layer 4 having the fineperiod lattice shape to realize a low reflectance (the minimumreflectance in the visible light region is 0.5% or less).

[0069] Note that even if an error (e.g., edge rounding or side surfacetilt) occurs in the shape of the protection layer 4 in actuallymanufacturing an element, antireflection performance similar to thatdescribed above can be realized by optimizing the filling factor andthickness of the protection layer 4.

[0070] As described above, according to this embodiment, by adding theprotection layer 4 onto the fine periodic structure 2 of the substrate3, the fine periodic structure 2 of the substrate 3 can be protectedagainst external environmental changes and the like. In addition, a verylow reflectance can be attained owing to the antireflection effectobtained by setting the refractive indexes of the incident-sidematerial, protection layer 4, and substrate 3 in the above relationshipand the antireflection effect obtained by the fine periodic structure ofthe protection layer 4 in addition to the antireflection effect obtainedby the fine periodic structure 2 of the substrate 3.

[0071] According to this embodiment, therefore, an antireflectionelement having both durability and excellent antireflection function canbe realized.

[0072] (Second Embodiment)

[0073]FIG. 4 shows the arrangement of an antireflection elementaccording to the second embodiment of the present invention. In anantireflection element 11, a fine periodic structure 12 on a substrate13 has a triangle section, and a protection film (additional layer) 14is formed along this triangle section.

[0074] The fine periodic structure has a periodic structure only in theone-dimensional direction (lateral direction on the drawing surface).FIG. 13 shows the grating shape of the substrate 13.

[0075] As a material for the substrate 13, PMMA was used, and a latticeperiod Λ, lattice width, and grating depth were respectively set to 0.2μm, 0.1 μm, and 0.19 μm.

[0076] A depth d2 of the protection layer 14 is preferably set to equalto or less than the grating depth dl from the viewpoint of ensuring therigidity of the protection layer 14.

[0077]FIG. 14 shows a comparison between the PMMA substrate itself andthe element having the fine periodic structure 12 with a trianglesection, formed on the surface of the substrate 13. Referring to FIG.14, the abscissa represents the wavelength; and the ordinate, thereflectance. The reflectance was calculated by using rigorouscoupled-wave analysis.

[0078] As is obvious from FIG. 14, by forming the fine periodicstructure 12 on the surface of the substrate 13, the reflectance isreduced.

[0079]FIG. 5 shows another comparison between the reflectance of theantireflection element having the protection layer 14 added to thesubstrate 13 so as to obtain a triangle section and others.

[0080] In this case, PMMA was used as a material for the substrate 13,and the grating period Λ, the grating width w, and grating depth d1 wererespectively set to 0.2 μm, 0.1 μm, and 0.19 μm. In addition, MgF₂(n=1.38) was used as a material for the protection layer 14, and theprotection film thickness d2 was set to 0.1 μm.

[0081] As is obvious from a comparison between a PMMA substrate alone(without antireflection film), an element having a single layerantireflection film (film material: MgF₂ (n=1.38, optical filmthickness: 125 nm) formed on a PMMA substrate, and an element having afine periodic structure with a triangle section formed on a PMMAsubstrate, the element having the protection layer 14 formed on the fineperiodic structure 12 with the triangle section exhibits the bestreflectance characteristics (lowest reflectance) and an improvement inthe reflectance of the substrate in the entire wavelength range.

[0082] Antireflection performance (low reflectance), which was difficultto realize by the only antireflection effects obtained by the singlelayer film alone and the fine periodic structure alone as in theconventional art, can be realized by optimizing the grating shape(grating period, grating depth, grating portion, and the like) of thefine periodic structure 12 on the substrate 13 and the shape (protectionfilm thickness, protection film material, and the like) of theprotection layer 14.

[0083] It is preferable that the refractive index of the material of theprotection layer 14 (not the effective refractive index but therefractive index of the material itself) be substantially equal to theaverage of the refractive indexes of the incident-side material (e.g.,air) and the material of the substrate 13. This makes the refractiveindex of the protection film material become intermediate between therefractive index of the incident-side material and the refractive indexof the substrate material. As a consequence, a refractive index changeis reduced, and the reflectance can be decreased.

[0084] In addition, the effective refractive index can be continuouslychanged from the incident side to the substrate side by forming theprotection layer 14 on the fine periodic structure 12 with trianglesection. As in the case of a sloping film in the conventional art,effects such as an increase in antireflection band can be expected.

[0085] As described above, according to this embodiment, the fineperiodic structure 12 on the substrate 13 can be protected againstexternal environmental changes by adding the protection layer 14 on thefine periodic structure 12 on the substrate 13. In addition, a very lowreflectance can be attained owing to the antireflection effect obtainedby setting the refractive indexes of the incident-side material,protection layer 4, and substrate 3 in the above relationship and theantireflection effect obtained by the fine periodic structure of theprotection layer 4 in addition to the antireflection effect obtained bythe fine periodic structure 12 of the substrate 13. Furthermore, thereflectance can be further decreased by optimally designing thethickness of the protection layer 14.

[0086] According to this embodiment, therefore, an antireflectionelement having both durability and excellent antireflection function canbe realized.

[0087]FIG. 6 shows the incidence angle dependency of reflectance.Referring to FIG. 6, the abscissa represents the wavelength; and theordinate, the reflectance. FIG. 6 shows the reflectance of aconventional single layer film and the reflectance of the antireflectionelement having the fine periodic structure 12 with the triangle sectionshape having the protection layer 14. Note that the incidence angle wasset to 0° and 15°.

[0088] As is obvious from FIG. 6, the incidence angle dependency of theantireflection element according to this embodiment stands comparisonwith that of the conventional single layer antireflection film. Inaddition, an antireflection element with reduced incidence angledependency of reflectance can be manufactured by optimizing the latticeshape and grating depth.

[0089] In the first and second embodiments described above, when a fineperiodic structure is to be formed on a substrate, the substrate neednot have a flat surface, and may have a curved surface like a lens ormirror or an uneven surface like a Fresnel lens.

[0090] The fine periodic structure in each embodiment described abovehas periodicity only in a one-dimensional direction. However, thepresent invention is not limited to this. For example, this element mayhave a two-dimensional fine periodic structure like a grating structurewhose grating shape is square or rectangular on an x-y plane. Inaddition, a protection film may be formed on an antireflection elementhaving a fine periodic structure whose grating period changes in threeor more directions like a grating structure whose grating shape ishexagonal on an x-y plane.

[0091] Each of the first and second embodiments has exemplified therectangular or triangle section of the fine periodic structure having anantireflection function. However, the present invention is not limitedto this shape. For example, the sectional shape of a fine periodicstructure may have any shape, e.g., a stepped shape, sawtooth shape,trapezoidal shape, sinusoidal shape, or semicircular shape, as long as aprotection film is added onto the surface of an antireflection elementhaving the fine periodic structure.

[0092] (Third Embodiment)

[0093]FIG. 7 shows an optical lens (optical element) according to thethird embodiment of the present invention. In this embodiment, a fineperiodic structure is integrally formed on the surface of an opticallens (base member) 5, and a protection film like the one described inthe second embodiment is added onto this fine periodic structure,thereby forming an optical lens having the fine periodic structure withthe protection film.

[0094] This makes it possible to realize a high-performance optical lenswhich protects the fine periodic structure on the lens surface and hasattained a decrease in reflectance.

[0095] Referring to this schematic view, since a fine periodic structure30 with a protection film is emphatically expressed, the size and shapeshown differ from those of the actual antireflection element portion. Inaddition, although the fine periodic structure is formed on only onesurface of the optical lens 5, such structures may be formed on both thesurfaces of the structure.

[0096] In an optical system (not shown) using the optical lens 5 withlittle Fresnel reflection on the surface, reductions in ghost and flaredue to light reflected by the surface of the optical lens 5 can beexpected. This technique is therefore especially useful for an opticalsystem using many optical elements, which is used in an opticalapparatus such as a camera, video camera, or liquid crystal projector.

[0097] Note that the optical element of the present invention is notlimited to the one according to this embodiment, but can be variouslyapplied within the range in which the basic function and performance arenot impaired. For example, a fine periodic structure substrate with aprotection film (or protection layer) formed as a discrete member may beintegrally boded on the surface of an optical lens or an antireflectionelement portion with a protection film may be formed on a prism-likeoptical element.

[0098] (Fourth Embodiment)

[0099]FIG. 8 shows the arrangement of an optical system according to thefourth embodiment of the present invention. This optical system is ascanning optical system used for a laser beam printer or the like.

[0100] Referring to FIG. 8, this system includes a light source 6 suchas a semiconductor laser, a collimator lens 7, a cylindrical lens 8, apolygon mirror 9, and an f-θ lens 10.

[0101] A fine periodic structure 40 with a protection film (protectionlayer) described in the first and second embodiments is formed on thesurface of the f-θ lens (base member) 10. This system also includes aphotosensitive member 11.

[0102] The fine periodic structure 40 with the protection film isintegrally formed on the f-θ lens 10 by forming a fine periodicstructure on the surface of the f-θ lens 10 and adding a protection film(or protection layer) on the fine periodic structure.

[0103] With this structure, an improvement in the durability of the fineperiodic structure on the surface of the f-θ lens 10 and a reduction inFresnel reflection can be expected.

[0104] Recently, demands have arisen for an increase in the resolutionof a laser beam printer, in particular. By reducing Fresnel reflectionon the surface of the f-θ lens 10, ghost appearing on the photosensitivemember 11 can be reduced, and an image output higher in resolution thanthat in the conventional art can be obtained.

[0105] In this embodiment, the fine periodic structure with theprotection film is formed on only one surface of the f-θ lens 10.However, such fine periodic structures may be formed on both thesurfaces of the f-θ lens 10, as needed.

[0106] As has been described above, a periodic structure with a periodsmaller than the wavelength of incident light is formed on the surfaceof a base member, and an additional layer having a refractive indexlower than that of the base member is formed on the periodic structure.With this structure, the additional layer is made to function as aprotection layer (or protection film) for the periodic structure on thebase member to protect the periodic structure on the base member againstenvironmental changes such as temperature and humidity changes.Furthermore, an element having an excellent antireflection function canbe realized by the combination of the antireflection effect obtained bythe fine periodic structure formed on the base member and theantireflection effect obtained because the refractive index of theadditional layer is lower than that of the base member.

[0107] In addition, if a periodic structure with a period smaller thanthe wavelength of incident light is formed as the above additionallayer, an element having a better antireflection function can berealized owing to the antireflection effect obtained by the periodicstructure of the additional layer itself in addition to the above twoantireflection effects.

What is claimed is:
 1. An element comprising: a periodic structureformed on a surface of a base member and having a period smaller than awavelength of incident light; and a layer added on said periodicstructure, said additional layer having a refractive index lower thanthat of a material for said periodic structure formed on the surface ofthe base member.
 2. An element according to claim 1, wherein saidperiodic structure formed on the surface of the base member decreasesreflectance on the base member.
 3. An element according to claim 1,wherein said layer has a periodic structure with a period smaller thanthe wavelength of incident light.
 4. An element according to claim 3,wherein the period of the periodic structure of said layer is equal tothe period of said periodic structure formed on the surface of the basemember.
 5. An element according to claim 1, wherein the period of saidperiodic structure formed on the surface of the base member is not morethan ½ the wavelength of incident light.
 6. An element according toclaim 1, wherein said additional layer has a thickness not more than adepth of said periodic structure formed on the surface of the basemember.
 7. An element according to claim 1, wherein an effectiverefractive index calculated by using said layer and an incident-sidematerial contacting said layer is higher than a refractive index of theincident-side material and lower than a refractive index of a materialfor said periodic structure formed on the surface of the base member. 8.An element according to claim 1, wherein an effective refractive indexcalculated by using said layer and an incident-side material contactingsaid layer is substantially equal to an average value of a refractiveindex of the incident-side material and a refractive index of a materialfor said periodic structure formed on the surface of the base member. 9.An element according to claim 1, wherein said layer protects saidperiodic structure formed on the surface of the base member.
 10. Anoptical member comprising: a base member; a periodic structure having aperiod smaller than a wavelength of incident light and formed on asurface of said base member; and a layer added onto said periodicstructure, said layer having a refractive index lower than that of amaterial for said periodic structure formed on the surface of the basemember.
 11. An optical system comprising said optical member defined inclaim
 10. 12. An optical device comprising said optical system definedin claim 11.